The present invention relates to the art of radio frequency interface circuits. The invention finds particular application in interfacing between the transmission and reception of magnetic resonance spectroscopy probe signals and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will find further application in other fields in which radio frequency signals are alternately transmitted and received including magnetic resonance imaging, magnetic resonance spectroscopic chemical or physical analysis, ultrasonics, radar, communications, and the like.
Heretofore, magnetic resonance spectrometers have commonly used linear polarization and crossed coils for simplicity of design. Single coil probes or antennas, which are used for both transmission and reception, require an interface circuit to connect the probe or NMR coil system to the transmitter during the transmit cycle and a receiver or preamplifier during the receive cycle. The interface circuits commonly included appropriate circuitry for isolating the transmitter and receiver at least during the transmission mode.
In the parent application hereto, a DC bias was applied to the interface circuit to effect switching between transmit and receive modes of operation. Although advantageous in many respects, the DC bias switching does have certain drawbacks. In particular, an external DC bias supply must be provided along with the associated components and circuitry to apply, isolate, and decouple the DC bias and the RF signal path. Moreover, precise timing is required to prevent the bias switching from occuring at an inappropriate time in the transmit/receive cycle. For example, if the DC bias was not applied to switch the interface circuit to the transmit mode before the application of RF power, RF power might be applied to the receiver. Additionally, the wave front from the DC switching or bias voltage may cause various undesirable transient effects, such as a broadband energy spike at the probe and preamplifier ports.
Other interface circuits have been switched between the transmit and receive modes with crossed pairs of diodes by utilizing the RF signal itself to bias the diodes between on and off states. Under the high energy levels of the transmit mode, the diodes were biased conductive; whereas under the lower energy of the receive mode, the diodes appeared as open circuits. Although the crossed diode switch designs eliminated the DC bias and the attendant bias control and interface circuitry, significant distortion of the RF wave form occured, generally known as crossover distortion. This distortion effect has been attributed to the intrinsic forward voltage drop of semi-conductor diodes typically about 0.7 volts for small signal types. Severity of the signal clipping generally exceeds the expected 1.4 volt peak-to-peak value because the RF power source did not see a proper load until the diodes were in the conducting state. In some instances, the available voltage output of the driving amplifier was greatly reduced until the diodes began to conduct, which clipped a substantial portion of the center of the RF wave form. In addition to the significant distortion, total elimination of signals at the lower end of the system's dynamic range resulted. For example, in NMR imaging and spectroscopy, the RF signals may have a wide dynamic range. Any distortion or clipping of the low level portions of the wave forms adversely impacted the quality of the results.
The present invention provides a new and improved interface which eliminates the above referenced problems and others to achieve improved switching of radio frequency signals.